Sniff test

The fluoroscopic sniff test, also known as diaphragm fluoroscopy, is a quick and easy real time fluoroscopic assessment of diaphragmatic motor function (excursion). It is used most often to confirm absence of muscular contraction of the diaphragm during expiration in patients with phrenic nerve palsy or breathing difficulties following stroke. Chest radiograph demonstrating a newly elevated hemidiaphragm often precedes a sniff test.

Technique

The following technique is suggested:

ask the patient to practice sniffing before the study

with the patient either standing (preferred) or supine, perform frontal fluoroscopy of the diaphragm at rest, breathing quietly through an open mouth

ask the patient to take a few quick short breaths in with a closed mouth ('sniffs') causing rapid inspiration

occasionally, repeating (3) in the lateral projection is required to evaluate the posterior hemidiaphragms

Findings

In normaldiaphragmatic motion:

the diaphragm contractsduring inspiration: moves downwards

the diaphragm relaxes during expiration: moves upwards

both hemidiaphragms move together

in healthy patients 1-2.5 cm of excursion is normal in quiet breathing 2

3.6-9.2 cm of excursion is normal in deep breathing 2

up to 9 cm can be seen in young or athletic individuals in deep inspiration 2

excursion in women is slightly less than men 2

In abnormaldiaphragmatic motion:

the affected hemidiaphragm does not move downwards during inspiration

paradoxical motion can occur

Interpretation

Absence of diaphragmatic movement confirms phrenic nerve palsy in the appropriate clinical setting. A mass anyway along the course of the phrenic nerve requires exclusion, usually with neck and chest CT. A hilar mass due to lung cancer is the most common finding on CT and a classic exam case.